In European patent application 1 188 783 A2 and U.S. Pat. No. 6,559,271, a process for making high IV PET in the melt phase is described. In this patent, high IV PET catalyzed with a titanium based compound is described as providing a good compromise between reactivity and selectivity when a low dosage of titanium metal and a low reaction temperature is chosen to obtain optimal increase in molecular weight and reduce the chance of thermal decomposition. By providing a more thermally stable polymer, the level of acetaldehyde (“AA”) generated in the polymer is reduced. The amount of AA generated by the described process in the base polymer is not stated, but after addition of an excess amount of AA bonding agent, the contemplated amount of AA in the polymer melt is described as ranging from 1 to 10 ppm directly after polycondensation. Recognizing that AA bonding additives can cause a stronger or weaker yellowing of the polyester polymer, the patent recommends controlling the color imparted by the AA reducing additives by adding bluing toners to the melt.
We have discovered that titanium catalyzed polycondensation reactions impart an unacceptably high yellow color to high It.V. base polyester polymers made in the melt phase as indicated by their high b*, a problem not addressed by U.S. Pat. No. 6,559,271. Adding sufficient amount of bluing toner to overcome the yellow color imparted to the melt by a titanium-catalyzed reaction presents the further problem of having to use higher amounts of bluing toners, which has the potential for reducing the brightness of the polymer and increases the costs for making the polymer composition.
In order to reduce the level of AA in the melt phase polymer, the process described in U.S. Pat. No. 6,559,271 operates the melt phase at a reduced temperature and with a reduced titanium catalyst concentration, i.e. low reaction temperature on the order of 270° C. and less than 10 ppm Ti metal as the catalyst concentration. However, by reducing the reaction temperature and catalyst concentration, the reaction time required to attain the same target molecular weight also increases.
It would be desirable to implement a solution to make a high KN. polymer in the melt phase with a better, lower b* (a measure of the yellow hue in the polymer). Moreover, it would also be desirable to retain the same or better, shorter reaction times to a target high It.V. in the melt compared to the reaction time needed to obtain the same target It.V. in titanium-catalyzed reactions with an acceptable b* color.